This invention relates to apparatus, systems and methods for processing and optimizing the power output generated by sources of energy where the voltage output varies substantially in amplitude and frequency as a function of time. A primary aim of the invention is to increase the net output power produced by such a source of energy.
Many sources of renewable energy, [e.g., wave energy converters (WECs), wind and solar] which are used to generate electrical energy, produce alternating signals whose amplitude and frequency vary over a wide range. This is most notable in ocean wave power generation systems where the rate (frequency) of the waves and their amplitude vary greatly; generally, in excess of those of most other applications. Accordingly, the invention will be discussed with respect to WEC systems for purpose of illustration. However, the invention is also applicable in other applications such as those using wind energy or solar thermal sources of energy, as well as applications such as electric vehicle regenerative breaking.
In general, the energy produced by, or present in, a source of renewable energy (e.g., ocean waves) is converted into mechanical energy which is used to drive an electrical generator (e.g., a rotary or linear generator). FIG. 1 is an illustrative example of a prior art wave energy converter system (WEC) which may be used to practice the invention. A spar 102 and a float 104 intended to be disposed in a body of water move relative to each other in response to the waves present in the body of water. A power take off device (PTO) 106 is coupled between the spar and float and includes apparatus to convert their relative motion into mechanical energy (force) which may be used to drive an electric generator (which may be included within the PTO, or external thereto) to produce a voltage output which, for purpose of illustration, may be as shown in FIG. 2.
FIG. 2 is a simplified profile of a possible voltage output generated by an electrical generator, as a function of time (t), driven by a WEC system of the type shown in FIG. 1. Note that for the periods from time t0 to t6, t7-t10, and t15-t18, the amplitude of the alternating voltage and/or current (providing power generation) is near the zero crossing point. As a result, for extended periods of time the corresponding power being generated is of very small amplitude.
It is known to rectify the alternating and variable output voltage of an electrical generator by means of a passive diode rectification system to charge a storage device such as a capacitor (or battery) to produce a DC voltage corresponding to the generator output. However, a passive diode system blocks conduction until the voltage at their anodes exceeds the voltage at their cathodes. So, for many conditions, a passive diode system is not effective to transfer charge from an electric generator to the storage device. Also, it is inefficient for the condition where the generator voltage output amplitude is not very large and a significant portion of the generated voltage and power is consumed in, or about, the diode rectifiers due to the voltage drops across the diodes.
To overcome these and other problems, it is preferable to use a power switching circuit, connected between the electric generator and a storage device, to rectify or convert the electric generators' alternating current (AC) output to a direct current (DC) voltage which is stored in the storage device. The power switching circuit may be an active current control pulse width modulation (PWM) circuit. The PWM circuit provides a much more controllable and consistent level of current control than passively rectified systems, and is capable of transferring energy from a low voltage generator output to a higher voltage DC bus
However, the power switching circuit introduces power losses. Most power switching circuits are switched at frequencies in the kilohertz range. This causes core losses in the electric generator through eddy current induction. The active switching also results in losses in the power switching circuits due to semiconductor switching and conduction losses. These losses are generally unavoidable and are always present.
Applicant recognized that operating the power switching circuitry connected between an electric generator and a storage device (to rectify the generator output voltage) when the generator's output voltage is insufficient to exceed the switching and core loss overhead results in a significant loss and waste of power.
The nature of the problem may be explained by reference to the highly simplified schematic diagram of FIG. 3 which shows a single phase alternating current (AC) generator 350. The output voltage (Eg) of generator 350 is generally cyclical about the zero axis varying generally at a frequency in the range of less than 1 to 60 Hz or more. A power switching circuit 352 is connected between the generator 350 and a capacitor Cx to convert (and transfer) the AC voltage generated by the generator 350 into a DC voltage stored by Cx. The power switching circuit 352 includes a switch SW1 connected across the generator coil and a switch SW2 connected between the generator 350 and capacitor Cx. Switches SW1 and SW2 (which are not turned on at the same time) are switched (turned) on and off at a rate of several KHz [i.e., the switch rate, which may be in the range of 1 KHz to More than 10 KHz, is much higher than the frequency of the voltage generator output (Eg)].
Applicant recognized that for low values of generator voltage (Eg), instead of power being transferred via SW2 to charge the capacitor Cx, power is in fact drained from capacitor Cx and flows via switch SW2 into the coil of generator 350 also causing heating and core losses. Thus, for low values of generator voltage, more power is consumed by the switching-driving system than is supplied, whereby there is a net power loss.
Applicant also recognized that in wave energy conversion systems, a significant percentage of the power generation time occurs near the zero crossings of the input power waveform and the output voltage is of low amplitude. For the low amplitude condition, the associated switching overhead and core losses will consume more power than is produced. Minimizing these parasitic losses can improve net power generation, especially in low wave states.
Applicant also recognized that wave power generation systems typically have a high ratio of peak power to average power. Since power generation equipment must be sized according to the peak power requirements, wave power systems tend to have very large electric generators and associated drives installed. These large electric generators and their associated drives consume substantial amount of power for the standby and low power operation conditions. As a result, the net power production and efficiency of the wave power system is significantly reduced.
For example, a system that produces 200 watts on a yearly average might have/need a 15,000 watt generator and drive to handle the peak power requirements. If that 15,000 watt generator and drive had a 50 watt no load standby loss, it would only represent a 0.3% loss for ordinary applications. However, for wave power, the standby loss can consume 25% of the average production and is much more of a problem. If there are no waves at all, the drive and generator system can be simply shut down. However, it will take some time to re-initialize the system if the waves return and operation is desired. On low wave activity days, efficiency can drop to very low levels—there may be enough waves to justify keeping the generation equipment powered on in standby mode, but the standby losses may consume nearly all of the power production.
An object of the invention is to optimize the efficiency of the electrical conversion process in those systems where the input power profiles are cyclical and/or variable.